There are substantial deposits of oil sands in the world with particularly large deposits in Canada and Venezuela. For example, the Athabasca oil sands region of the Western Canadian Sedimentary Basin contains an estimated 1.3 trillion barrels of potentially recoverable bitumen. There are lesser, but significant deposits, found in the U.S. and other countries. These oil sands contain a petroleum substance called bitumen (similar to an asphalt) or heavy oil (a highly viscous form of crude oil). Oil Sands deposits cannot be economically exploited by traditional oil well technology because the bitumen or heavy oil is too viscous to flow at natural reservoir temperatures.
Often the oil sands deposits may be tilted such that some of the resource will be found near the surface but much of the resource will occur at ever greater depths of burial. This is the case, for example, in the Athabasca oil sands of Alberta, Canada.
When oil sand deposits are at or near the surface, they can be economically recovered by surface mining methods. Recovery by surface mining is economical when there is, at most, a relatively thin layer of overburden that can be removed by large surface excavation machines. In current state-of-the-art oil sands surface mines, the exposed oil sands are excavated directly by large power shovels, transported by large haulage trucks to a conversion facility called a cyclofeeder. The ore is crushed and turned into a slurry in the cyclofeeder. From there, the slurry is hydrotransported to a large extraction facility where the bitumen is separated from the ore. The bitumen recovered from the extraction process is then transported to an upgrader facility where it is refined and converted into crude oil and other petroleum products.
The Canadian oil sands surface mining community is evaluating machines that can excavate material at an open face and process the excavated oil sands directly into a slurry. If such machines are successful, they could replace the shovels and trucks and cyclofeeder facility currently used, by producing an oil sands slurry at the working face which could then be sent via a hydrotransport system to a bitumen extraction facility.
In the large surface mining process described above, there is substantial disturbance of the surface. In Canada especially, the disturbed surface must be returned to its original condition after the recovery operations are complete. This requirement adds significantly to overall bitumen recovery costs. In the large surface mines, excavating the material and extracting the bitumen contribute significant emissions (principally carbon dioxide and methane) to the atmosphere.
When oil sand deposits are too far below the surface for economic recovery by surface mining, bitumen can be economically recovered in many areas by recently developed in-situ recovery methods such as SAGD (Steam Assisted Gravity Drain) or other variants of gravity drain technology which can mobilize the bitumen or heavy oil. The in-situ methods require a certain level of overburden for the process to be contained and also require deposits of a certain minimum thickness (typically greater than about 20 meters). The recovery factor of the in-situ methods can be degraded by the presence of intervening mud and shale layers within the deposits which can form barriers to the outward flow of steam and return flow of mobilized bitumen or heavy oil. Thus the economics of these processes are sensitive to the complex and variable natures of the reservoir geologies that are found. In the SAGD method, horizontal drilling technology is used to drill two closely spaced horizontal wells near the bottom of the ore deposits. These well pairs are used to inject steam into the formation above to heat and mobilize the bitumen. The heated bitumen then flows downward by gravity and is collected in one of the horizontal wells and pumped to the surface. The bitumen is then processed and sent to an upgrader facility.
SAGD requires enormous amounts of energy to generate steam to heat the underground deposits to the point where the bitumen can flow and be pumped. Typically, 20% to 30% of the energy recovered from a barrel of bitumen must be used to produce the steam required to recover the next barrel of bitumen in the SAGD process. The production of energy to produce steam also contributes significantly to greenhouse gas emissions.
Roughly 65% (approximately 845 billion barrels) or most of the deposits in the Athabasca cannot be recovered by either surface mining or in-situ technologies. There is a considerable portion of oil sands deposits that are in “no man's land”. These are areas where either (1) the overburden is too thick and/or there is too much water-laden muskeg for economical recovery by surface mining operations; (2) the oil sands deposits are too shallow for SAGD and other thermal in-situ recovery processes to be applied effectively; or (3) the oil sands deposits are too thin (typically less than 20 meters thick) for efficient use of surface mining or in-situ methods. This “no man's” land also includes significant deposits within the surface mineable areas that are under too much overburden, under swamps or under large tailings ponds. These “no man's” land deposits within the surface mineable areas are significant and contain tens of billions of barrels of economic grade bitumen. There is currently no viable means to recover the bitumen or heavy oil from these “no man's” land areas. Estimates for economical grade bitumen in these “no man's” land areas range from 30 to 100 billion barrels.
These “no man's” land deposits can be exploited by an appropriate underground mining technology. One such underground mining technique is the use of large soft-ground tunneling machines which are designed to backfill most of the tailings behind the advancing machine. This concept is described in U.S. patent application Ser. No. 09/797,886, filed Mar. 5, 2002, and entitled “Method and System for Mining Hydrocarbon-Containing Materials”, which is incorporated herein by this reference. By this method, an ore slurry, such as produced by the cyclofeeder facility of a surface mine, or a bitumen froth, such as produced by a SAGD operation, can be outputted by the backfilling Tunnel Boring Machine or TBM, depending on whether any substantial ore processing is done inside the TBM. The material used for backfilling most of the volume excavated is provided by processed spoil or tailings from which the hydrocarbon or valuable ore has been extracted.
One embodiment of the mining method envisioned by U.S. patent application Ser. No. 09/797,886 involves the combination of slurry TBM excavation techniques with hydrotransport haulage systems as developed by the oil sands surface mining industry. A TBM operated in slurry mode can be designed to produce an oil sands slurry compatible with the density requirements of an oil sands hydrotransport system. Such a system appears to be capable of efficiently excavating oil sands, transporting the oil sand slurry to the surface for processing and then hydrotransporting a tailings slurry back to the advancing TBM for use as backfill material. TBMs may also be operated in non-slurry or dry mode. When operated in dry cutting mode, the TBM may still be a fully shielded machine with full isolation of the excavated material from the manned interior of the TBM and its trailing tunnel liner. In another embodiment of the mining method envisioned by U.S. patent application Ser. No. 09/797,886, the bitumen may be separated inside the TBM or mining machine by any number of various extraction technologies.
The Athabasca oil sand is a dense interlocked skeleton of predominantly quartz sand grains with pore spaces occupied by bitumen, water, gas and minor amounts of clay. The sand grains are whetted by water and the bitumen does not directly contact the grains. The bitumen is a semi-solid hydrocarbon substance resembling asphalt. Because the bitumen is semi-solid and very viscous, it causes the oil sand to be relatively impermeable to the flow of free water and gas. Gas is present as discrete bubbles and also dissolved in both the bitumen and water.
For example, at 150 meters of overburden, it has been estimated that 0.3 to 0.6 cubic meters of gas is dissolved in a cubic meter of oil sand mined. This gas is typically composed of 80% methane and 20% carbon dioxide. When exposed to atmospheric pressure, the dissolved gas comes out of solution and can be released into the atmosphere, for example by surface mining. Methane is a powerful greenhouse gas which is estimated to be equivalent to 21 times its weight as potent as carbon dioxide.
For the purposes of the present invention, the entities referred to variously as lumps, particles and matrices in the published art are referred to as granules, to distinguish them on one hand from sand grains or particles which they contain, and on the other hand from large lumps of oil sand as mined. Such granules include a nucleus of sand grains covered with a film of connate water, which may itself contain fine particles, encapsulated, often with gas inclusions, within a layer of the heavy oil known as bitumen, which is essentially solid at ground temperatures. The terms oil and bitumen are used interchangeably in this specification.
The process originally developed for releasing bitumen from oil sands was the Clark hot water process, based on the work of Dr. K. A. Clark, and discussed in a paper “Athabasca Mineable Oil Sands: The RTR/Gulf Extraction Process—Theoretical Model of Detachment” by Corti and Dente which is incorporated herein by reference.
Both the presently used commercial method and apparatus for the recovery of oil or bitumen from oil sands based on the Clark process, and the similar process and apparatus described in U.S. Pat. No. 4,946,597, use vigorous mechanical agitation of the oil sands with water and caustic alkali to disrupt the granules and form a slurry, after which the slurry is passed to a separation tank for the flotation of the bitumen from which the bitumen is skimmed. As proposed in the U.S. patent, the process may be operated at ambient temperatures, with a conditioning agent being added to the slurry. Earlier methods, such as the Clark process, used temperatures of 85° C. and above together with vigorous mechanical agitation and are highly energy inefficient. It is characteristic of both of the above processes that a great deal of mechanical energy is expended on physically disintegrating the oil sands structure and placing the resulting material in fluid suspension, this disintegration being followed by physical separation of the constituents of the suspension. Chemical adjuvants, particularly alkalis, are utilized to assist these processes. The separation process particularly is quite complex, as will be readily apparent from a study of U.S. Pat. No. 4,946,597, and certain phases have presented particularly intractable problems. Oil sands typically contain substantial but variable quantities of clay, and the very fine particles constituting this clay are dispersed during the process, limiting the degree to which the water utilized in the process can be recovered by flocculation of the clay particles. No economical means has been discovered of disposing of the flocculated and thickened clay particles, which form a sludge which must be stored in sludge ponds where it remains in a gel-like state indefinitely.
The Clark process has disadvantages, some of which are discussed in the introductory passage of U.S. Pat. No. 4,946,597 which is incorporated herein by reference, notably a requirement for a large net input of thermal and mechanical energy, complex procedures for separating the released oil, and the generation of large quantities of sludge requiring indefinite storage.
The Corti and Dente paper mentioned above suggests that better results should be obtained with a proper balance of mechanical action and heat application, and Canadian Patent No. 1,165,712, which is incorporated herein by reference, points out that more moderate mechanical action will reduce disaggregation of the clay content of the sands. Nevertheless, it continues to regard external mechanical action as playing an essential role in the disintegration of the oil and granules, which will inevitably result in partial dispersion of the clay. Thus, it proposes to use relatively more gentle agitation of the sand in a slowly rotating digester described in Canadian Patent No. 1,167,238 which is incorporated herein by reference. The digester in Canadian Patent No. 1,167,238 comprises in its broadest embodiment a shell, means for entry of liquids and solids into the shell at one end of the shell, a tubular outlet at the other end of the shell for discharge of liquids, a solids outlet at the same end as the liquids outlet, surrounding but separated from the liquids outlet, and a screw which surrounds the tubular liquids outlet to urge solids to and through the solids outlet, which screw is secured at its outer periphery to the shell. As seen in FIGS. 1, 2, 3 and 4 of Canadian Patent No. 1,167,238, the operating embodiment of the digester includes numerous plates and bars secured to the shell for moving the solids along the shell, and a set of bars for separating the clay from the oil sands. Slurry is introduced at one end of the shell. This slurry is a mixture of oil sands and hot water. The slurry is moved by the plates, bars and screw down the shell during which it is agitated and the oil and water gradually separated from the solids. At the other end of the shell, such oil and water, together with some fine material that has separated from the solids, is removed from one central, axial outlet, while the solids exit the digester at its base. This process, which is a concurrent process, still requires considerable post digestion treatment, as described in Canadian Patent No. 1,165,712. The post digestion steps include further separation of the liquids into an oil rich component and a middlings component consisting primarily of water and fines, removing the fines from the middlings component by flocculation and centrifuging, and further treating the oil rich component for the removal of contained water, fines and solids. A detailed outline of the process is described with reference to FIG. 1 of Canadian Patent No. 1,165,712.
Separator cells, ablation drums, and huge interstage tanks are typical of apparatuses necessary in oil sands extraction. The one with perhaps the greatest potential is the Bitmin drum or Counter-Current De-Sander system or CCDS. Canadian Patent 2,124,199 provides a method of liberating and separating heavy oil or bitumen from oil sand in a counter current desanding apparatus known as a bitmin drum. The bitmin drum is a rotating vessel with various internal fins and pockets into which oil sand ore is fed at the upstream end and water is fed in at the downstream end. The outputs of the bitmin drum are a bitumen froth (bitumen, water and some sand and clay) slurry and a separate damp sand discharge.
Rather than seeking to find a balance of thermal and mechanical action to release the oil from the sand, Canadian Patent 2,124,199 relies mainly on thermal action alone to provide release or liberation of the bitumen. The presence of hot water acts as a medium both for heat transfer and for separation to occur. Mechanical action is used to ensure adequate contact between the water and the oil sand and its separated constituents so as to permit it to act effectively as both a heat transfer medium and a separation medium. The action of the bitmin drum is described in detail in Canadian Patent 2,124,199 and other references which are hereby incorporated by reference in the present invention.
The CCDS process is carried out in the bitmin drum, comprising submerging sand to be treated into a bath of hot water, gently rolling the sand within the bath. The resultant agitation of the water is sufficient to prevent liberated oil droplets from migrating to the surface of the bath, and the rolling of the sand is gentle enough to minimize substantial dispersion of any clay present. It is, however, sufficiently prolonged to permit substantial release and separation of oil coating from granules of the sand, removing sand from one end of the bath, and removing water, and oil from the other end of the bath. The sand and hot water are supplied at opposite ends of the bath to those at which they are removed. By passing the oil sand to be treated and the hot water in opposite directions through the bath, various advantages accrue. For example, separated oil froth passes with the water towards the opposite end of the bath from that at which the separated sand is removed, thus minimizing the risk of re-entrainment of oil on the sand as the latter is removed. The sand is exposed to the hottest water in the later stages of its treatment, thus favoring completion of liberation of the oil and the separation process. A settling zone may be provided at the end of the bath from which the oil is removed, thus again favoring separation of the suspended solid particles from the water and oil before the latter leaves the bath.
An important objective of the CCDS process is to minimize the attrition of clay lumps in the oil sands with resultant suspension of clay solids in the treatment water. This is achieved by minimizing mechanical working of the oil sands during the release and separation process. The less clay is suspended, the easier is the treatment and recycling of the water used in the process, and the less clay sludge is produced requiring indefinite storage. An objective is to leave most of the clay essentially in its original state so that it may be returned, together with the separated sand, to the site from which the raw oil sands were extracted.
Other oil sands extraction methods include, but are not limited to, cyclo-separators in which centrifugal action is used to separate the low specific gravity materials (bitumen and water) from the higher specific gravity materials (sand, clays etc). The cyclo-separator has a number of major disadvantages including but not limited to (i) the need to comminute large rocks and remove contaminants, such as wood and tramp metal from input streams to avoid damaging the cyclo-separator; (ii) high rates of equipment wear and the concomittant need to use expensive abrasion resistant materials; (iii) de-aeration of the recovered bitumen which causes problems for downstream stages of separation; and (iv) cyclone failure or viscous plugging due to a black froth condition for high bitumen content ores. All studies to-date have led to the abandonment of the hydro-cyclone solution, even in very large fixed separation facilities.
The TCS process is a variant of the cyclone method, which involves three cyclones in a counter-current backwash configuration. The TSC circuit, as presently conceived, is a very large device because of the large front-end rougher separator cell which heads up that circuit.
Commercial surface mining operations in the oil sands require the excavation, haulage and processing of vast amounts of material. Once the bitumen has been extracted, the volume of tailings is actually greater than the original volume. This is because the bitumen originally resides in the pore space of interlocked sand grains. Even with the bitumen removed, the sand grains cannot be reconstituted into their original volume even under tremendous pressure. Thus, current surface mining methods result in a large and costly tailings disposal problem.
In a mining recovery operation, the most efficient way to process oil sands is therefore to excavate and process the ore as close to the excavation as possible. If this can be done using an underground mining technique, then the requirement to remove large tracts of overburden is eliminated. Further, the tailings can be placed directly back in the ground thereby eliminating a tailings disposal problem. The extraction process for removing the bitumen from the ore requires substantial energy. If a large portion of this energy can be utilized from the waste heat of the excavation process, then this results in less overall greenhouse emissions. In addition, if the ore is processed underground, methane liberated in the process can also be captured and not released as a greenhouse gas.
There is thus a need for a bitumen/heavy oil recovery method in oil sands that can be used to perform one or more of the following functions: (i) extend mining underground to substantially eliminate overburden removal costs; (ii) avoid the relatively uncontrollable separation of bitumen in hydrotransport systems; (iii) properly condition the oil sands for further processing underground, including crushing; (iv) separate most of the bitumen from the sands underground inside the excavating machine; (v) produce a bitumen slurry underground for hydrotransport to the surface; (vi) prepare waste material for direct backfill behind the mining machine so as to reduce the haulage of material and minimize the management of tailings and other waste materials; (vii) reduce the output of carbon dioxide and methane emissions released by the recovery of bitumen from the oil sands; and (viii) utilize as many of the existing and proven engineering and technical advances of the mining and civil excavation industries as possible.